The term "positive feeding" defines a feeding condition during which the yarn feeding device determines the feeding speed of the yarn fed to a textile machine, in particular to a knitting machine. The term "non-positive feeding" is a well-known technical expression defining an operational state of the yarn feeding device under which the yarn can be freely withdrawn from the yarn feeding device at a yarn speed which is determined by the actual demand of yarn of the textile machine.
Yarn feeding devices for selectively positively feeding or non-positively feeding of yarn to a knitting machine are commonly known in the art. For example, U.S. Pat. No. 3,908,921 discloses a yarn feeding device for a knitting machine comprising a rotatable yarn storage drum onto which a yarn is wound for forming an intermediate store of yarn and from which the yarn can be unwound and fed to the knitting machine. The prior art yarn feeding device comprises a movable yarn control element which is disposed adjacent to the withdrawal path of the yarn near the withdrawal end of the storage drum. This movable yarn control element can be brought into a first position in which it defines a fixed withdrawal point of the yarn withdrawn from the storage drum so as to limit the yarn withdrawal speed to the tangential speed of the storage drum. In a second position of the movable yarn control element it does not come into contact with the unwinding yarn so that the yarn can be freely withdrawn from the storage drum at a withdrawal speed which is determined by the actual demand of yarn of working elements in the knitting machine. This prior art yarn feeding device can only be equipped with a rotating storage drum as the rotational speed of the storage drum in the first mode of operation for the positively feeding of yarn to the textile machine determines the yarn feeding speed. In the second mode of operation for the non-positively feeding of yarn to the knitting machine, the rotational speed determines the amount of yarn wound on the storage drum per time unit. Hence, the increasing or decreasing of the rotational speed of the storage drum during this second mode of operation results in an increasing or decreasing store of yarn on the drum. When changing the mode of operation from the positive feeding state to the non-positive feeding state, an abrupt adjustment of the rotational speed of the storage drum becomes necessary. The abrupt changing of the rotational speed of the rotatable storage drum causes problems due to the high inertia of the storage drum.
In view of this state of art, the present invention is based on the technical task of creating a yarn feeding device adapted for the selectively positively or non-positively feeding of yarn to a textile machine having a good dynamic behaviour.
In accordance with the present invention, the yarn feeding device is equipped with a stationary yarn storage drum, as known per se in the art (EP-A-107 110) and comprises a rotatable yarn-guiding member arranged at the withdrawal end of the storage drum. The rotatable yarn-guiding member is driving at a rotational speed which depends on the operational speed of the textile machine at least during the time intervals of the positive feeding of yarn to the textile machine. The rotatable yarn-guiding element comprises at least one yarn engagement element adapted for engaging the yarn when the rotational speed of the yarn spiralling around the withdrawal end of the storage drum tends to exceed the rotational speed of the yarn-guiding member so as to define a yarn feeding speed during these time intervals of positive feeding. The yarn engagement element is adapted for coming out of engagement with said yarn so as to allow a free withdrawal of yarn from the storage drum independent from the rotational speed of the guiding member when the rotational speed of the guiding member exceeds the rotational speed of the yarn spiralling around the withdrawal end of the storage drum.
In one embodiment, the first rotational speed is chosen such that the desired positive yarn feeding speed is obtained. The second rotational speed is chosen such that it exceeds the first rotational speed.
In a second embodiment, the rotating yarn guiding member can be continuously driven at a fixed rotational speed if the textile machine requires a yarn feeding speed under positive feeding conditions which exceeds the actual yarn demand of the textile machine under non-positive yarn feeding conditions.